Superchargers are widely used as an auxiliary device to provide high combustion energy by an internal combustion engine. For example, an exhaust gas turbine-type supercharger (i.e. turbocharger) is configured to compress air to be supplied to an internal combustion engine by causing exhaust gas from the internal combustion engine to rotate a turbine rotor to obtain a driving force and by causing a compressor impeller to rotate by using the driving force.
A hybrid turbocharger, which is a type of superchargers, has an electric generator. In a hybrid turbocharger, the rotor of the electric generator and the compressor impeller are connected to each other via a coupling member to constitute the impeller assembly. With such a supercharger, electric power is obtained by rotating the generator by the driving force of the turbine rotor obtained from excessive exhaust gas energy.
A power assisted turbocharger, which is another type of superchargers, has a built-in electric motor. In a power assisted turbocharger, the rotor of the electric motor and the compressor impeller are connected to each other via a coupling member to constitute the impeller assembly. With such a supercharger, the compressor is driven by supplementarily using the driving force of the electric motor when the amount of the exhaust gas to drive the turbine is small, for example, at the time of low load operation of the internal combustion engine.
Patent Document 1 discloses a power assisted turbocharger having a rotor overhang structure where the rotor of the electric motor is supported in the manner of the cantilever on the tip portion of the shaft which is inserted in the compressor impeller.
In the power assisted turbocharger disclosed in Patent Document 1, the electric motor is mainly composed of a motor rotor, a stator and a casing. Among them, the motor rotor is a member having a cylindrical structure and having a magnet portion on its outer circumferential side, and one end portion of the motor rotor is connected to the tip portion of the shaft inserted in the compressor impeller by means of a flange coupling. That is, a flange member mounted on the tip portion of the shaft and a flange portion provided on one end portion of the motor rotor are joined to each other with a plurality of bolts and nuts.